The present invention relates to scintillator crystals, to a manufacturing process allowing them to be obtained and to the use of the said crystals, especially in gamma-ray and/or X-ray detectors.
A scintillator crystal is a crystal which is transparent in the scintillation wavelength range which responds to incident radiation by emitting a light pulse. Scintillator crystals are widely used in detectors for gamma-ray, X-rays, cosmic rays and particles whose energy is of the order of 1 keV and greater. From such crystals it is possible to manufacture detectors in which the light emitted by the crystal that the detector comprises is coupled to a light-detection means and produces an electrical signal proportional to the number of light pulses received and to their intensity. In scintillation devices the detector is generally a single scintillator crystal.
Solid state scintillator crystals are in common use as components of radiation detectors in X-ray detection apparatus such as counters, image intensifiers and computerized tomography (CT) scanners. Such detectors are used especially in the fields of nuclear medicine, physics, chemistry and oil well logging. One embodiment of the present generation of scintillators comprises oxide mixtures in which a rare earth oxide is present as an activator, along with various combined matrix elements which are also usually rare earth oxides. Other combined metals may also be present as additives for specific purposes. These scintillators have been characterized by the advantageous properties of high efficiency, moderate decay time, low afterglow and little or no radiation damage upon exposure to high X-ray doses.
A family of known scintillator crystals widely used is of the thallium-doped sodium iodide, or NaI:T1, type. Crystals of the NaI:T1 family have a low density and therefore a low detection efficiency for certain types of high-energy radiation; they also have hygroscopic problems.
Another family of scintillator crystals is of the barium fluoride (BaF2)type. Crystals of the BaF2 family are not very dense and their rapid emission component lies within the ultraviolet range, which means the use of expensive photodetectors in scintillation devices.
Another family of scintillator crystals which has undergone considerable development is of the bismuth germanate (BGO) type. Crystals of the BGO family have a long scintillation decay time which limits the use of such crystals to low counting rates.
A more recent family of scintillator crystals was developed in the 1980s and is of the cerium-activated gadolinium orthosilicate (GSO) type. Crystals of the GSO family have a low optical yield and a strong tendency to cleave, which makes them extremely difficult to prepare.
A new family of crystals was developed at the end of the 1980s in order to obtain scintillator crystals having a high light yield, short luminescence decay times and a high detection efficiency: these crystals are of the cerium-activated lutetium oxyorthosilicate (LSO) type and formed the subject-matter of patent U.S. Pat. No. 4,958,080. A method of growing such a crystal formed the subject-matter of patent U.S. Pat. No. 5,660,627. Although the scintillation properties of the crystals of this family are excellent, they do have a major drawback with regard to reproducibility, which has a negative impact on the development of their use. This is because the results of scintillation properties between two crystals of the same composition may vary very considerably as indicated, for example, by the following publications: xe2x80x9cCe-doped scintillators: LSO and LuAPxe2x80x9d (A. Lempicki and J. Glodo, Nuclear Instruments and Methods in Physics Research A416 (1998), 333-344) and xe2x80x9cScintillation Light Emission Studies of LSO Scintillatorsxe2x80x9d (A. Saoudi et al., IEEE Transactions on Nuclear Science, Vol. 46, No. 6, December 1999). These authors indicated in particular the difficulties of using LSO owing to very large variations in the scintillation properties of LSO single crystals from one crystal to another, even when they are cut from the same ingot.
Another drawback with LSO relates to its high melting point, about 2200xc2x0 C. and this means that the process allowing such a crystal to be obtained requires high temperatures.
The latest scintillator compositions employ at least one of the oxides of lutetium, yttrium and gadolinium as matrix materials. These are described in detail, for example, in U.S. Pat. Nos. 4,421,671, 4,473,513, 4,525,628, 4,783,596, and 6,093,347. These crystals typically comprise a major proportion of yttria (i.e., Y2O3), up to about 50 mole percent gadolinia (Gd2O3) and a minor activating proportion of a rare earth activator oxide. Suitable activator oxides, as described in the aforementioned patents, include the oxides of europium, neodymium, ytterbium, dysprosium, terbium and praseodymium. Europium-activated scintillators are often used in commercial X-ray detectors by reason of their high luminescent efficiency, and low afterglow level. Decay times of such scintillators are on the order of 0.9-1.0 millisecond.
The search thus continues for scintillator compositions having improved properties.
The object of the present invention is to alleviate these drawbacks and to propose a novel family of scintillator crystals whose scintillation properties are of the same order of magnitude as those of LSO crystals, wherein the property variations from one crystal to another of the same composition are very much less than the property variations from one LSO crystal to another of the same composition.
One crystal according to the invention is a monoclinic single crystal obtained by crystallization of a congruent molten composition of general formula:
LU2(1-x)M2xSi2O7
where LU is selected from lutetium, or a lutetium-based alloy which also includes one or more of the elements Sc, Y, In, La, Gd; where M is cerium, or cerium partially substituted with one or more of the elements of the lanthanide family (excluding lutetium); and where x is a variable defined by the limiting level of Lu substitution with M in a monoclinic crystal of the lutetium pyrosilicate (LPS) structure.